pasting content from another cell

In tiled rendering case (headless) each window has its own clipboard,
otherwise exists a unique clipboard (UNO service).

Change-Id: I7d5d0b085faeaffa3fc0a80914fbe9349f4aa402
Reviewed-on: https://gerrit.libreoffice.org/56508
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>